Vehicles are equipped with a transmission adapted to convert power generated by an internal combustion engine and to provide output power in accordance with traveling conditions. There are, for example, gear type transmissions and belt type transmissions. The gear type transmission is widely used because power transmission loss is less.
One example of such transmissions is shown in FIG. 5 wherein the transmission 202 comprises: a main shaft 204 for inputting power from an internal combustion engine (not shown); a main first-speed gear 206, a main reverse gear 208, a main second-speed gear 210, a main third-speed gear 212 and a main fourth-speed gear 214 all functioning as a main gear train arranged in a predetermined way on the main shaft 204; a counter shaft 216 disposed in parallel relation with the main shaft 204; a counter first-speed gear 218, a counter reverse gear 220, a counter second-speed gear 222, a counter third-speed gear 224 and a counter fourth-speed gear 226 all functioning as a counter gear train arranged in a predetermined way on the counter shaft 216; a final reduction drive gear 228 mounted on the counter shaft 216; a final reduction driven gear 232 meshed with the final reduction drive gear 228 and next connected to a differential mechanism 230; a transfer drive gear 238 meshed with the final reduction driven gear 232 and mounted on an intermediate shaft 236 of a transfer mechanism 234; a transfer drive bevel gear 240 mounted on the intermediate shaft 236; a transfer driven bevel gear 244 mounted on a transfer driven pinion shaft 242 in such a manner as to mesh with the transfer drive bevel gear 240; a companion flanged shaft coupler 246 connected to the transfer driven pinion shaft 242; a synchronous meshing mechanism 248 mounted on the counter shaft 216 disposed between the counter third-speed gear 224 and the counter fourth-speed gear 226; and other component elements.
Also, the transmission 202 of FIG. 5 further includes an extremely low speed gear mechanism 250 for reducing the speed to a range even lower than the first-speed gear train comprising the main first-speed gear 206 and the counter first-speed gear 218 as a lowest gear train. The extremely low speed gear mechanism 250 includes a main shaft extremely low speed gear 252 mounted on one end portion of the main shaft 204, a rear extremely low speed idler gear 256 meshed with the gear 252 and mounted on an extremely low speed idler shaft 254, a forward low speed idler gear 258 fixedly connected to the idler gear 256, a main shaft forward low speed idler gear 260 meshed with the forward low speed idler gear 258 and rotatably mounted on the main shaft 204, a counter shaft forward low speed gear 262 meshed with the main shaft forward low speed idler gear 260 and rotatably mounted on the counter shaft 216, a reverse low speed idler gear 264 fixedly connected to the extremely low speed idler gear 256, a counter shaft reverse low speed gear 266 meshed with the reverse low speed idler gear 264 and rotatably mounted on the counter shaft 216, and a forward/reverse shift mechanism 268 rotatably mounted on the counter shaft 216 and axially slidably disposed between the counter shaft forward low speed gear 262 and the counter shaft reverse low speed gear 266.
Examples of known transmissions are disclosed in Japanese Patent Publication No. Sho 63-39450 and Japanese Patent Early Laid-open Publication No. 62-278339. The transmission disclosed in Japanese Patent Publication No. Sho 63-39450 is constructed such that when an extremely low speed gear train is established by a synchronous meshing mechanism, driving force from an input shaft is transmitted to an output shaft at a great reduction ratio via the extremely low speed gear train to obtain an extremely low speed, an intermediate shaft and a secondary shaft being disposed in parallel relation with the input and output shaft axes, a secondary transmission mechanism being juxtaposed to a main transmission mechanism in order to restrain increase of the overall length of the transmission, a secondary shaft and an intermediate shaft of the secondary transmission mechanism each having one end supported on the end wall o a clutch case, whereby the secondary transmission mechanism, as a whole, is disposed in such a manner as to be one-sided to a portion of a transmission case, so that the transmission case is increased in rigidity and formed compact in size.
The transmission described in Japanese Patent Early Laid-open Publication No. Sho 62-278339 is designed such that gears EL.sub.1 to EL.sub.4 and a synchronous mechanism, which are axially supported in parallel relation with a main shaft and a drive pinion shaft, are disposed in a space where the gear trains of an ordinary transmission are arranged, and therefore, the entirety can be properly laid out without necessity for a large transmission case.
In a transmission, the moment of inertia of parts to be synchronized has a great effect on the synchronization efficiency as a basic efficiency of the transmission.
However, in the transmission 202 shown in FIG. 5, the extremely low speed gear mechanism 250 is directly mounted on the main shaft 204 and as the axes and gears of the extremely low speed gear mechanism 250 are rotated in accordance with rotation of the main shaft 204, the synchronization efficiency is impaired with the increased moment of inertia caused by these parts. Any attempt to increase, for example, the length of the tapered portion where the synchronizer ring of the synchromesh mechanism contacts in order to maintain the synchro efficiency at a favorable level fails to decrease the moment of inertia to a satisfactory extent, thus failing to enhance the synchro efficiency.
Therefore the present invention attempts to obviate the above-mentioned inconveniences by providing, a transmission with an ultra low speed range which is capable of maintaining the synchro efficiency at a favorable level without using a large sized synchromesh mechanism and which can effectively prevent generation of gear rattling sounds by eliminating rotation of the shafts and gears of the extremely low speed gear mechanism when in the neutral position.
According to a preferred embodiment of the present invention, there is provided a transmission with an ultra low speed range in which the transmission gear ratio can be changed in accordance with the meshing state between a main shaft gear train mounted on a main shaft and a counter shaft gear train mounted on a counter shaft and which is provided with an extremely low speed gear mechanism adapted to reduce the speed to a range even lower than the lowest gear train. The transmission comprises an extremely low speed shaft provided with an extremely low speed main gear train of said extremely low speed gear mechanism and disposed coaxially with said main shaft, a first shift mechanism having a first changeover sleeve which is moved in order to bring said extremely low speed shaft into or out of engagement with said main shaft, and a second shift mechanism having a second changeover sleeve which is moved for selectively shifting a forward low speed gear train and a reverse low speed gear train into driving engagement between the extremely low speed shaft and the counter shaft.
According to the construction of the present invention, when in an extremely low speed range, firstly the first changeover sleeve of the first shift mechanism is moved in order to connect the main shaft with the extremely low speed shaft, and because of the fact that the change speed pattern is in a forward position or in a reverse position of the extremely low speed range, the vehicle is moved forwardly or reversely in an extremely low speed range.
On the other hand, as the first changeover sleeve is moved in such a manner as to separate the extremely low speed shaft from the main shaft when in a normal traveling range, the shafts and gears of the extremely low speed gear mechanism are not rotated and, as a result, synchro efficiency can be maintained at a favorable level without increasing the moment of inertia, and generation of gear rattling sound can be effectively prevented when in this neutral position.